1. Field of the Invention
The present invention relates to surface treatment of plastics and, more particularly, to the formation of a durable antistatic coating on the surface of acrylic resins such as polymethylmethacrylate.
2. Description of the Prior Art
A very desirable property is exhibited by solid acrylic polymers, such as crystal clarity, high impact strength, formability, and resistance to sunlight and weather in most chemicals has led to widespread use in military and civilian applications, especially since these polymers have become available in transparent, translucent and brightly colored forms. Acrylic sheet is the standard transparent material for canopies, windows, instrument panels, searchlight and landing light covers on aircraft. Acrylics are finding increasing use in automotive fields, as a sign material for faces and letters, display and demonstration models, vending machines and industrial and architectural applications such as windows, safety shields, etc.
The problem with acrylic plastics is that an electrostatic charge easily builds up on the surface of the article. A particularly serious problem is that a charge can build up on aircraft canopies or windows during flight to such an extent as to cause fracture. The presently known and available antistatic treatments are neither sufficiently durable nor do they provide sufficient charge dissipation without loss of optical clarity. Treatment of the acrylic surface with polyionenes which were previously found to act as antistatic agents because of their high positive charge density was found to be unsuccessful in the case of acrylic polymers. A commercial antistatic agent such as DuPont's Zelec-NK was found to disspate static electricity and yield good clarity when applied to the surface of acrylic articles. However, the agent could be easily removed by rubbing or washing. Strong electron acceptors such as tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), and dinitrobenzene were studied and were found to form charge transfer complexes with polymethylmethacrylate which increased the conductivity markedly and dissipated static electricity. However, these agents severely attacked the surface yielding poor optical clarity.